I can think of a long way to do it.
48961- (0.025*48961)=A(customer base after 1 year)
A-(0.025*A)=B(customer base after 2 years)
B-(0.025*B)=C(3 years.....)
and so on. There is probably an easier way to solve this with n! or something, but I don't know off hand.

^^ that's basically the way to do it
Since it's continuously declining, you can also use: \(\large y=Ce^{kt} \)
where C is your initial number of customers and k = -0.025.
Plug in t=13 and you should get 35375.66